1. Field of the Invention
The present invention relates generally to proximity sensors and more particularly relates to sonic proximity sensors adapted to sense targets, such as people and animals, which are located behind a vehicle.
2. Description of the Prior Art
The problem of detecting warm bodies, such as people and animals, in an area behind a vehicle is well established. The problem is especially acute in the field of farm tractors and lawn mowing equipment. Presently, more than five thousand injuries occur each year as a result of misuse of such equipment. A large percentage of these injuries are inflicted when a tractor is backing up and inadvertently strikes an unseen victim, resulting in impact injuries and lacerations from contact with a spinning lawn mower blade.
Various forms of electronic sensors have been employed in collision avoidance systems to detect targets in the path of a moving vehicle. These sensor types include radio-based sensors (radar), ultrasonic sensors, laser-based sensors and passive infrared sensors. Each of these sensor types has associated advantages and disadvantages. For example, radar-based systems have the ability to detect the position and relative speed of a target in the vicinity of a vehicle. However, such sensors are costly and are most responsive to hard, reflective surfaces such as other vehicles, buildings and the like. Infrared sensors, such as passive infrared sensors which are well known in the art, employ pyroelectric or thermopile detectors to sense the movement of a warm body in the sensor's field of view. Such sensors are commonly used in security systems to detect the presence of an intruder within a protected premise and have also been used in robotic systems to detect and track people in the vicinity of a mobil robotic system. While passive infrared sensors can discriminate between a warm body, such as a person, and inanimate objects, these sensors cannot determine the range of a target from a vehicle. Passive infrared sensors are also subject to false detection of a warm body due to thermal variation within the sensor's field of view.
It is well established that sonic sensors can be employed to detect the presence of a target in the proximity of a vehicle. Such technology, commonly referred to as sonar, transmits a burst of sonic energy which will reflect off a target proximate to a vehicle. After transmitting the burst of sonic energy, the sonar system enables a receiver to detect any reflected sonic energy. As the distance of a target is proportional to the time that it takes for the sonic energy to reach a target and return to the receiver, such sonar systems are capable of not only determining the presence of a target, but also can accurately render the distance of the target with respect to the vehicle.
The use of sonic sensors to detect obstacles in a region behind a moving vehicle is also known in the art. For example, the article "Limitations of Ultrasonic Obstacle Sensors for Industrial Lift Truck Applications," by Walter J. Girardi, published by SAE International, SAE Technical Paper Series No. 961809, discloses the use of an ultrasonic sensor on a lift truck to reduce accidents involving pedestrians and moving industrial lift vehicles. The system disclosed employs a single ultrasonic sensor mounted on a center portion of the rear of a lift truck. This system provides a conical protection area behind the vehicle and does not provide 180.degree. coverage behind the lift truck. Accordingly, the system disclosed does not provide any coverage in the region directly to the left or the right of the rear of the vehicle, which are within the vehicles immediate turning radius. Therefore, while it is known to use ultrasonic sensors to detect the targets behind the vehicle, there are shortcomings with those devices known in the prior art and there remains a need for a back-up proximity sensor system for a vehicle which overcomes these problems.